Ametropia, an undesirable refractive condition of the eye, has three main subdivisions: myopia, hyperopia, and astigmatism. In myopia, by far the most common type of ametropia, the parallel light rays 1 which enter the eye as shown in FIG. 1(c) come to a focus F3 in front of the retina 2. In hyperopia, the rays of light 1 come to a focus F2 behind the retina 2 as shown in FIG. 1(b). When the rays of light converge to not one, but several foci, it is referred to as astigmatism, in which condition the various foci may all lie before the retina; all lie behind the retina; or partly before and partly behind the retina.
Ametropia is usually corrected by glasses or contact lenses. However, these refractive disorders may also be corrected by surgery. Refractive eye surgery is defined as that surgery on the eye that acts to change the light-bending qualities of the eye. More common current refractive procedures include radial keratotomy, as described in U.S. Pat. Nos. 4,815,463 and 4,688,570 and also laser ablation of corneal stroma, described in U.S. Pat. No. 4,941,093. Various other surgical methods for the correction of refractive disorders have been tried including thermokeratoplasty for the treatment of hyperopia, epikeratoplasty to correct severe hyperopia, and keratomnileusis, which can steepen or flatten the central cornea. Keratomileusis was introduced by Barraquer of Colombia in 1961 and essentially involves grinding a corneal button into an appropriate shape to correct the refractive error and replacing the reshaped corneal button. Some of the more commnon keratorefractive procedures are discussed below; none of which have all the characteristics of an ideal keratorefractive procedure. The disadvantages of corneal refractive surgery include limited predictability, lack of reversibility, corneal destabilization, optical zone fibrosis, post-operative discomfort, and visual symptoms such as glare, halos, and starbursts.
In radial keratotomy (RK), multiple peripheral radial incisions are made into the cornea at 90-95% depth in an attempt to flatten the central cornea and thus correct myopia. The problem of unpredictability of result was tackled by multiple extensive retrospective analyses of the patients in whom surgery had already been performed. These studies revealed certain factors that seemed to control the outcome of the surgery, such as the size of the optical zone, the initial keratometric readings, corneal diameter, corneal rigidity, number of incisions, incision depth, intra-ocular pressure, thickness of the cornea, and degree of astigmatism. Age and sex are also factors that are taken into consideration in most of the nomograms, which have been devised to predict what effect to expect for a certain surgery. At one point, many experts in the field considered it nearly impossible to fully and accurately correct patients in one surgery and felt that RK should be considered a two-stage surgery, with the initial surgery to achieve the "ball-park" correction, followed by an enhancement procedure to adjust or titrate the result near the desired outcome for an individual eye. It was felt that because of individual variability which may lead to an under or over-correction in the individual different from that predicted by the nomogram attempting to fully correct the refractive error in one surgery could lead to over-correction in a significant number of surgeries, resulting in hyperopia which is much more difficult to correct. Unfortunately, the second-stage surgery is even less predictable than the initial procedure. No one has yet devised a formula to take into account the profound changes which occur in the cornea after the initial RK, especially when weeks or months have passed. Most studies quote only 50-60% of eyes achieving 20/20 or better visual acuity following RK. Patients who are accustomed to 20/20 or better corrected visual acuity before surgery are not typically satisfied with less than 20/25 or 20/30 uncorrected post-operative visual acuity.
In addition, a gradual hyperopic shift is a major concern after RK. Refractive stability is critical for all refractive procedures but all corneal refractive procedures show significant degrees of instability. To date, there has been no clear explanation of why the cornea is destabilized by RK. A recent report on the long-term results of RK stressed the "natural" hyperopic refractive progression of "normal" eyes as a function of age. It is possible that patients are initially overcorrected and the over-correction masked by the patient's accommodative powers. With time and loss of accommodation, the hyperopia may be gradually unmasked with the hyperopia becoming visually symptomatic. At the time of surgery, a patient may be corrected with resultant slight hyperopia and yet have 20/20 vision because of the ability of the lens to accommodate. There is a range of residual correction within which the patient can have 20/20 uncorrected vision. This range varies depending on the individual but probably spans two to three diopters. Even with this range, the percentage achieving 20/20 is only 50-60%. This reflects poorly on the precision of the technique. It is important to note that this range diminishes with presbyopia, or loss of accommodation which usually begins at about 45 years of age. This results in the percentage achieving 20/20 dropping from the 50-60% described above. It is obvious that RK does not qualify as a simple, safe, predictable procedure to adjust the refractive outcome after the initial RK has been performed. Most ideas to contend with the corneal shape after this event have been purely empirical. Thus an easy method to fine-tune a refractive correction that is minimally invasive and easily performed would require serious consideration.
Laser stromal ablation procedures, such as photorefractive keratectomy (PRK) for correction of refractive disorders are currently popular and have had reasonable success. These procedures are not, however, spared from the problem of unpredictability. Essentially, in the treatment of myopia, laser energy is imparted to the central cornea thereby causing excision of more tissue centrally and a resultant flattening of the cornea. Unfortunately, the final refractive effect is determined not only by the amount of ablation but also by the healing response to the keratectomy. The cornea actively lays down new collagen and the epithelium undergoes a hyperplastic response, among other responses, in an attempt to repair the damage to its surface. This causes regression, or a shift backwards towards myopia, which can gradually occur over a period of months to years. An undesired effect of new collagen deposition is stromal scar formation which manifests as stromal haze and possible decrease in contrast sensitivity by the patient. This corneal stromal opacification is variously referred to as fibrosis, scarring, or haze which is associated with reduced visual acuity and contrast sensitivity, regression of the refractive effect, and poor night vision. Predictability with PRK is an issue, as with RK. Most published results of outcome after PRK treatment for myopia show 80-94% of eyes obtaining uncorrected visual acuity of 20/40 or better while the percentage of patients achieving 20/20 is significantly less. These numbers are in spite of the fact that there is a range of residual refraction at which the patient can still see 20/20 as previously explained. It can be assumed that a significant proportion of those achieving 20/20 after PRK are actually slightly hyperopic. It may very well be that with time, a significant percentage of those patients develop "progressive hyperopia", or an unmasking of the latent hyperopia. So, although the percentage of patients achieving 20/20 after PRK is not acceptable by the definition of an ideal refractive procedure, it may be inflated as was the initial results with RK. Although visual recovery is slow in RK, it is quicker than after PRK. A second laser ablation procedure is usually undertaken with caution since it may cause a greater healing response with even more regression than the initial procedure. Again, as in RK, the laser ablation procedure is not completely predictable, partly because one cannot predict an individual's wound healing response.
The criteria for an ideal refractive procedure include adjustability, predictability/efficacy, maintenance of quality of vision, reversibility, simplicity, stability, safety, and low cost. Each of these factors are reviewed:
1. Adjustability. Because of corneal wound healing and the delicate corneal curvature changes required to achieve 20/20 uncorrected vision, it is unlikely that any refractive procedure will achieve 20/20 vision with a single procedure. The refractive literature is abundant with comments and opinions describing the need for an adjustable procedure. PRK attempts adjustments or "enhancements by repeating the same process--removal of epithelium and re-ablation. If there was variability in the initial PRK procedure, it is unlikely that the same procedure will be able to "fine-tune" the refractive outcome. PA0 2. Predictability/Efficacy. The bottom line is that only about 71% to 97% of eyes with baseline refractions less than or equal to 6 D achieve manifest post-op refractions within +/-1 D of the attempted correction one year after surgery. PRK, RK, and corneal ring literature all report similar efficacy data. PA0 3. Quality of vision. PRK ablates the central cornea and corneal haze is an issue as evidenced by a small percentage of patients with a decrease in best-corrected visual acuity. Visually comprising complications include haze and scarring, halo effect, decreased contrast sensitivity, and decentration of ablation. Any procedure that operates on the central cornea will result in decreasing the best-corrected visual acuity in at least a small percentage of patients. PA0 4. Reversibility. Many prominent refractive surgeons believe that the trend in refractive surgery would e towards reversible procedures. A reversible procedure, such as corneal rings, may likely be the procedure of choice for lower myopia and a reversible procedure, such as implantable intraocular lenses, may be the procedure of choice for higher myopia. Many potential patients have not had vision correction surgery because of its irreversibility. PA0 5. Simplicity. A refractive procedure that only a few skilled refractive surgeons can perform will unlikely become a popular procedure. Also, difficult procedures typically have variable outcomes and a steeper learning curve. The complication rate for beginning refractive surgeons for a difficult procedure cannot be dismissed since there will always be new surgeons learning the procedure. PA0 6. Stability. A long term complication of Radial Keratotomy is progressive hyperopia. PRK undergoes regression of effect. PA0 7. Safety. Refractive surgeries are elective procedures and much of the initial resistance to performing these procedures in the late 80's was the philosophy among ophthalmologists that surgically manipulating a healthy eye with the potential for disastrous loss of vision was ethically unacceptable. PRK can cause central corneal haze sufficient to decrease best-corrected visual acuity. LASIK invades a central cornea with a microkeratome and is associated with multiple potential complications including lost flaps, button-holed flaps, free caps, thin flaps, and even perforation with extrusion of intraocular contents. In general, a corneal procedure is probably safer than an intraocular procedure and a peripheral corneal procedure safer than a central corneal procedure.
For well over a century, ophthalmologists have searched for a surgical method to permanently correct refractive errors. At least 15 different techniques have been developed and considerable experience has accumulated in both animal and human models. Laser photorefractive keratectomy has come the closest to gaining widespread acceptance in the ophthalmic community, but the difficulty in gaining acceptance for keratorefractive procedures is because of the unsolved problems with poorly predictable and unstable refractive outcomes, adverse effects on the quality of vision, lack of adjustability, and irreversibility.
Poor predictability looms as the largest unsolved problem with refractive corneal surgery. The two major factors that contribute to poor predictability are (a) the variability and inaccuracy inherent with manual surgical techniques, and (b) the variable influence of corneal wound healing in determining the refractive outcome. Until these two deficiencies are corrected, it is unlikely that a refractive surgical procedure will predictably correct ametropia to within a half diopter, the margin that can be achieved routinely with glasses or contact lenses.
Photorefractive keratectomy offers the possibility of solving one of the major causes of poor predictability by reducing the surgical variability of the procedure. A major unresolved issue is how the second nemesis that causes poor predictability, corneal wound healing, will affect the results of PRK. However, patients typically undergo regression of effect of approximately 1.0-1.5 diopters over six months. 9%-20% of patients continued to show myopic regression of 1 D or more even during the second year after surgery. There are "under-responders" who do not undergo any regression and there are over-responders who undergo up to 3 diopters of regression. So, although it is true that the excimer laser is precise to sub-micron accuracy, the variability in regression removes that advantage.
Typical refractive surgery studies, including RK and PRK, report the post-operative refraction in terms of the percentage of patients achieving +/-1 diopter of emmetropia. Approximately 80-90% of patients achieve this range. However, only approximately 50-60% of patients achieve uncorrected visual acuity of 20/20 without symptoms. The unsolved problem in refractive surgery is that only about half of patients achieve a vision of 20/20 without correction following current refractive procedures.
The goal in refractive surgery is to achieve emmetropia. However, there is a range of residual refractive error at which the patient can see 20/20 without correction. On the myopic side, a patient can be -0.30 D or less and still see 20/20 uncorrected. The issue is slightly more complex on the hyperopic side due to the availability of lens accommodation. The average 30 year-old has a total of 7 diopters of accommodation available to him and can easily supply several diopters from his "storehouse" of reserve. Reading a book or newspaper at arms-length (40 cm) requires 2.5 diopters of accommodation for the emmetrope. This means that a 30 year old patient may be overcorrected by up to 2.5 diopters and still see 20/20 uncorrected when tested, with minimal symptoms for distance vision since he is using only 2.5 diopters of the 7 D available to him. Of course, now to read, he will require 5.0 diopters of accommodation and will most likely be unable to read comfortably. In the ages of 35-40, the 1.50 diopter hyperope who could always get along easily without wearing his correction for distance vision will suddenly find that he cannot.
The modem refractive surgeon has several weapons in his armamentarium to choose from in attacking myopia. The refractive surgeon knows the limitations of his options. It is understood that RK is moderately predictable, adjustable only towards hyperopia, and irreversible. PRK is also moderately predictable, adjustable only towards hyperopia with the caveat that there is some regression towards myopia, but also essentially irreversible. On cardinal rule of refractive surgery is to avoid overcorrection because the options for a patient who is over-corrected to hyperopia are much more limited.
The dilemma results from the surgeon (and patient) wishing to achieve an uncorrected visual acuity of 20/20. The uncorrected visual acuity is poor if the post-op refraction is myopic but 20/20 if the post-op refraction is hyperopic. However, residual myopia can be "enhanced" while residual hyperopia is much more difficult to surgically manage.
This is best illustrated in a study comparing Summit and Visx lasers. The results showed a median refraction of 0.0 D in the Summit group and -0.5 D in the Visx group. The uncorrected visual acuity was 20/40 or better for 100% of the Summit treated eyes, whereas only 85% of the Visx treated eyes achieved 20/40 or better. Overcorrection results in a higher percentage of patients achieving 20/40 or better but results in a higher percentage of patients who are hyperopic. In other words, the higher percentage of patients achieving 20/40 in the Summit group may be explained by the accommodative reserve still present in the younger patients that were overcorrected.
For years it has been thought that refractive surgery with intracorneal implants could be used in the correction of ametropia. Early techniques included lamellar removal or addition of natural corneal stromal tissue, as in keratomileusis and keratophakia. These required the use of a microkeratome to remove a portion of the cornea followed by lathing of either the patient's (keratomileusis) or donor's (keratophakia) removed cornea. The equipment is complex, the surgical techniques difficult, and most disappointingly, the results quite variable. The current trend in keratorefractive surgery has been toward techniques that are less traumatic to the cornea, that minimally stimulate the wound healing response, and behave in a more predictable fashion. The use of alloplastic intracorneal lenses to correct the refractive state of the eye, first proposed in 1949 by Jose Barraquer, have been plagued with problems of biocompatibility, permeability to nutrients and oxygen, corneal and lens hydration status, etc. Other problems with these lenses included surgical manipulation of the central visual axis with the concomitant possibility of interface scarring.
More recent efforts toward the correction of refractive errors have focused on minimizing the effects of the wound healing response by avoiding the central cornea. There have been multiple attempts to alter the central corneal curvature by surgically manipulating the peripheral cornea. These techniques are discussed because of their specific relevance to this invention.
Zhivotovskii, D. S., USSR Patent No. 3,887,846, describes an alloplastic, flat, geometrically regular, annular ring for intracorneal implantation of an inside diameter that does not exceed the diameter of the pupil. Refractive correction is accomplished primarily by making the radius of curvature of the surface of the ring larger than the radius of curvature of the surface of a recipient's cornea in order to achieve flattening of the central area of the cornea. Surgical procedures for inserting the ring are not described.
A. B. Reynolds (U.S. Pat. No. 4,452,235) describes and claims a keratorefractive technique involving a method and apparatus for changing the shape of the optical zone of the cornea to correct refractive error. His method comprises inserting one end of a split ring shaped dissecting member into the stroma of the cornea, moving the member in an arcuate path around the cornea, releasably attaching one end of a split ring shaped adjusting member to one end of the dissecting member, reversibly moving the dissecting member about the path, and thereby pulling the adjusting member about the circular path, made by the dissecting member, withdrawing the dissecting member, adjusting the ends of the split-ring shaped adjusting member relative to one another to thereby adjust the ring diameter to change the diameter and shape of the cornea and fixedly attaching the ring's ends by gluing to maintain the desired topographical shape of the cornea.
A major advantage of this ring was that a very minimal wound healing effect was expected. A marked corneal wound healing response would decrease the long-term stability of any surgical refractive procedure. However, there are two distinct problem areas affecting the refractive outcome of surgical procedures treating ametropia:
1. The first problem is concerned with the ability to predetermine the shape and size of an implant that will lead to a certain refractive outcome. In RK or PRK, retrospective studies have been performed that led to the development of nomograms which predict that a certain depth cut or a certain ablation amount will result in a predictable amount of correction. In the case of the ring, eventually nomograms will be developed that can be used to predict a given refractive correction for a given thickness or size of the ring. However, these nomograms can never fully account for individual variability in the response to a given keratorefractive procedure. PA1 2. The refractive outcome also depends on the stability of the refractive correction achieved after surgery. To reiterate, the advantage of the ring would be the stability of the refractive outcome achieved because of a presumed minimal wound healing response. This decreases the variability of the long-term refractive outcome but still does not address the problems posed in the first problem area, --the inherent individual variability, in that while the outcome may be stable, it may very well be an inadequate refractive outcome that is stable. PA1 Another unaddressed issue is that even with the implant, surgeons will aim for a slight under-correction of myopia because, in general, patients are more unhappy with an overcorrection that results in hyperopia. Again, the refractive outcome may be more stable than in RK or PRK but it may be an insufficient refractive result that is stable.
Simon in U.S. Pat. No. 5,090,955 describes a surgical technique that allows for modification of the corneal curvature by inter-lamellar injection of a synthetic gel at the corneal periphery while sparing the optical zone. He does discuss an intra-operative removal of gel to decrease the volume displaced and thus adjust the final curvature of the central corneal region.
Siepser (U.S. Pat. No. 4,976,719) describes another ring-type device to either flatten or steepen the curvature of the cornea by using a retainer ring composed of a single surgical wire creating a ring of forces which are selectively adjustable to thereby permit selective change of the curvature of the cornea,--the adjustable means comprising a turnbuckle attached to the wire.
There are several mechanisms by which peripheral manipulation of the cornea affects anterior corneal curvature. The cornea, like most soft tissues, is nonlinear, viscoelastic, nonhomogeneous, and can exhibit large strains under physiologic conditions. The whole eye is geometrically extremely complex and the biomechanics technique capable of systematically modeling this reality is the finite element method which assumes small strains (a measure of deformity), homogeneity, and linear elastic behavior. Two simple mechanisms will be briefly described.
A simple example is helpful in understanding the first mechanism. Assume a loose rope R between two fixed points P1 and P2 as in FIG. 2(a), which forms a curve, the lowest point P being in the middle. Referring to FIG. 2(b), a weight w placed on the rope between the middle point P and one fixed point will cause the central portion of the rope to straighten 11. The cornea C demonstrated in FIG. 3(a) and FIG. 3(b) behaves similarly, the two fixed points, P1 and P2, analogous to the limbus of the eye and the weight W similar to the intrastromal implant 16 which, when inserted in the cornea in surrounding relation to the corneal central optical zone, causes the corneal collagen fibers to deviate around the implant. In essence, this deviation of the cornea around the peripheral implant caused by volume displacement in the peripheral cornea results in other areas of the cornea losing "slack", or relatively straightening 14.
Mechanical expansion of a corneal ring's diameter also flattens the central corneal curvature whereas constriction of the diameter steepens the central corneal curvature. A constricting or expanding implant is likely to cause a less stable refractive outcome because the inward or outward forces of the implant against the corneal stroma may gradually cause further lamellar dissection and dissipation of the forces. A more consistent outcome is likely to be achieved with varying the volume displaced in the peripheral cornea as described by Simon.
The second mechanism is aptly described by J. Barraquer in the following quote. Since 1964, "It has been demonstrated that to correct myopia, thickness must be subtracted from the center of the cornea or increased in its periphery, and that to correct hyperopia, thickness must be added to the center of the cornea or subtracted from its periphery." Procedures involving subtraction were called `keratomileusis` and those involving addition received the name of `keratophakia`. Intrastromal corneal ring add bulk to the periphery and increasing the thickness of the ring results in a more pronounced effect on flattening of the anterior corneal curvature by "increasing (thickness) in its periphery".
In the February, 1991 issue of Refractive and Corneal Surgery, T. E. Burris states that "the thickening effects of ICR implantation may prove most important for maintenance of corneal fattening" and that "new ICR designs must take into consideration thickness effects on corneal flattening.
Again, the ideal keratorefractive procedure allows all the advantages of eyeglasses or contact lenses, namely, being able to correct a wide range of refractive errors, accuracy or predictability, allowing reversibility in the event that the refractive state of the eye changes and it becomes necessary to adjust the correction again, yielding minimal complications, and associated with technical simplicity, low cost, and being aesthetically acceptable to the patient. The goal of refractive surgeons should be to achieve 20/20 uncorrected visual acuity with long-term stability in greater than 95% of patients. None of the currently available refractive surgery procedures generate this degree of accuracy or stability.
An easy procedure to post-operatively fine-tune the refractive correction and corneal curvature which is often influenced by changes in corneal hydration status, wound healing responses, and other unknown factors, is not available. Each of the techniques described suffers from a limited degree of precision. In this disclosure of the present invention, an easy method to adjust the refractive outcome after the corneal curvature has stabilized, a method that is minimally invasive, a method causing minimal stimulation of the wound healing processes, allowing repetitive adjustments as deemed necessary, and being almost completely reversible is described. It may make moot the pervasive issue of unpredictability and make obsolete the application of procedures which rely heavily upon nomograms to predict refractive outcome and are thus unable to adequately account for an individual's variable response to the procedure.